The present invention relates generally to the installation of underground utility lines and more particularly to an arrangement for monitoring the tension or force applied to a utility line as it is installed through an underground bore such that exposure of the utility line to damaging levels of tension can be avoided.
The use of horizontal boring tools for the purpose of installing, for example, underground utility lines is increasingly popular due to several factors including recent improvements in horizontal boring tool guidance technology, As an example, see copending U.S. application Ser. No. 08/835,834 entitled SYSTEMS, ARRANGEMENTS AND ASSOCIATED METHODS FOR TRACKING AND/OR GUIDING AN UNDERGROUND BORING TOOL which is incorporated herein by reference. In the installation of an underground line using a boring tool, a pilot bore is typically drilled from the location of a drill rig to a pit or location at the far end of the drill run. It should be appreciated that drilling of the pilot hole may be accomplished with either guided or unguided boring tools. In either case, following drilling of the pilot bore, the drill string remains present in the pilot bore. In most instances, a method is employed in which the pilot bore is drilled having a diameter which is smaller than the diameter of the line to be installed. Thereafter, a reaming operation is performed in which the bore is brought to a suitable size while the utility line is installed. Several advantages are attendant to this method. In a first advantage, drilling a large borehole is avoided since it is known that large drill heads are difficult to steer. In a second advantage, since the drill rig is pushing on the drill string during drilling (i.e., creating compression), the amount of force which may be applied to the drill string is limited by problems such as, for example, buckling of the drill string. In contrast, during the reaming operation, substantial pulling force may be applied to the drill string since the latter is under tension. A typical prior art reaming process will be described in detail immediately hereinafter.
Referring to FIG. 1, a prior art reaming apparatus, generally indicated by the reference numeral 12, is being used in the installation of a utility line 14 in a previously drilled pilot bore 16. Utility line 14 may comprise, for example, a gas line or an electrical cable. A drill string 18 remains in pilot bore 16 following the drilling operation. A reamer 20 is typically installed on drill string 18. The reamer includes a plurality of nozzles and/or cutting picks (none of which are shown). As mentioned previously, drilling of pilot bore 16 is accomplished with a drill head (not shown) installed on the drill string. After reaching the far end of the drilling run, the boring tool is accessed (usually in a pit) and removed from the drill string. Reamer 20 is then installed on the drill string. Line 14 is connected with reamer 18 in any suitable manner using, for example, a Kellums grip 22 and a swivel arrangement 24. Many other suitable arrangements may be found for use in attaching the line to the reamer.
Still referring to FIG. 1, during installation of line 14, drill string 16 is retracted towards the drill rig in the direction indicated by an arrow 26 while the drill string rotates in a suitable direction as indicated by an arrow 28. Of course, reamer 20 is rotated and advanced by the associated actions of the drill string. At the same time, a drilling slurry (not shown) is pumped down drill string 18 and is ejected from the nozzles present on the reamer. The drilling slurry serves to (1) provide cutting action as it is ejected from the nozzles, (2) lubricate reamer 20, (3) provide cooling, (4) lubricate line 14, (5) provides soil stability and (6) viscously support removed soil (not shown) in a mixture with the slurry and is ultimately ejected from the borehole. Advancement of the reamer, in turn, pulls utility line 14 through the reamed pilot bore while swivel 24 serves to isolate the line from the rotation of the reamer.
Having described the manner in which an underground utility line is installed in accordance with the prior art, it should be appreciated that a significant problem may be encountered. Specifically, as mentioned, the drill rig is capable of exerting substantial pulling force on the drill string during the reaming/installation operation. In the event, for example, that the utility line binds within the pilot bore, the utility line may itself be exposed to a excessive portion of the pulling force. In fact, the utility line may be subjected to tension levels which result in damage to the line. Such damage may result in premature failure of the line or in breaking the line at some unknown underground location. In either case, the entire drilling and installation process usually must be repeated at significant expenditure.
As will be seen, the present invention provides a highly advantageous, heretofore unseen tension monitoring arrangement and associated method which solves the foregoing problems and which provides still further advantages.